This invention relates to temperature sensing devices. More particularly, the invention relates to a temperature sensing device in which the temperature sensing element may be replaced separately from the surrounding structure.
Many industrial processes require that the temperatures of the material being processed be monitored closely at various points. For example, it may be necessary or desirable to obtain temperature readings of a substance contained within a reaction vessel or flowing through a conduit. Temperature readings from within a vessel or conduit may be obtained from a temperature sensing device positioned in a thermowell structure extending into the conduit or vessel from a conduit or vessel wall. As shown in FIG. 1, a traditional thermowell structure 10 comprises a base or mounting detail 12 which could be fixed within a conduit or vessel wall 14 and a closed-ended sheath 16 extending a desired distance into the conduit or vessel. The temperature sensing device 18 used with the thermowell included an elongated probe 20 with a sensing element 22 sealed in the end 24 which was inserted into the thermowell sheath to the desired position. Leads 26 from the sensing element carry temperature data signals from the sensing element to suitable signal processing and control equipment.
The structure shown in FIG. 1 provided the benefit of allowing the temperature sensing device 18 to be replaced without disrupting the process being monitored. A drawback of the thermowell and separate temperature sensing device, however, related to the separate and replaceable nature of the temperature sensing device as well. The temperature sensor element within the temperature sensing device was separated from the medium to be sensed by both the sheath wall of the thermowell and the probe wall of the temperature sensing device itself. Even though the materials used for the thermowell sheath wall and temperature sensor probe wall had good thermal conductivity properties, the mass necessarily associated with the two separate walls of material caused the sensor element to respond slowly to temperature changes in the medium being sensed.
Alternatively to using a thermowell and separate temperature sensor, a "direct immersion" temperature sensing device may be fixed in the wall or a vessel or conduit with an elongated probe extending into the vessel or conduit. As compared to the thermowell and separate sensing device shown in FIG. 1, this direct immersion type sensing device eliminated one layer of material between the sensor element and the medium being sensed. The reduced mass of the direct immersion type sensing device allowed the device to respond more quickly to temperature changes and thus provided more accurate temperature readings. However, replacing a failed direct immersion type sensing device required that the process being monitored be completely shut down and the vessel or conduit opened when the failed device was removed. This interruption of the process being monitored was always time-consuming and expensive and was even impossible in some circumstances.
Another problem with both direct immersion sensors and thermowell temperature sensing arrangements related to the number of different sizes of temperature sensing devices required for monitoring a given process. Both the direct immersion sensors and the thermowell arrangements shown in FIG. 1 are of varying lengths depending upon the particular application. For example, thermowells mounted upon relatively small conduits are usually shorter than thermowells mounted on relatively larger conduits. Each length of thermowell required a correspondingly length temperature sensing device. At least one replacement sensing device was usually stocked for each length of thermowell resulting in high inventory costs. The same sort of inventory was required for direct immersion sensors.